The present invention relates generally to a composite vacuum chamber for use in containing a particle beam and more particularly to a vacuum chamber for such use in a rapidly changing magnetic environment. This invention is the result of a contract with the Department of Energy (Contract No. W-7405-ENG-36).
Particle beams produced in accelerators generally have to travel in a vacuum. This means that the travel path for the particle beam must be within an enclosure, such as a vacuum chamber. The vacuum chamber, of course, must be strong enough to withstand the inward press of the atmosphere. Metal pipes are strong enough to withstand atmospheric pressure. However, metal pipes introduce magnetic field perturbations due to their electrical conductivity. Furthermore, the rapidly changing magnetic environment induces eddy-currents within the metal of the pipe. Given enough strength and repetitive change in the magnetic field, these eddy-currents can heat the pipes sufficiently to deform or completely destroy the integrity of the seal against the atmosphere, in addition to drawing power from the magnetic field. On the other hand, the vacuum chamber must have conductivity in order to stabilize the beam. For this, a metal pipe is more than adequate.
Ceramic pipes have also been known as being strong enough to preserve a vacuum within and not be deformed by the pressure of the atmosphere. However, the ceramic pipe is an insulator and, hence, is unable to serve as a conductive pathway for rf currents. Thus, when the particle beam travels within a ceramic vacuum chamber, image currents are not able to travel along the chamber walls. This will cause instabilities in the particle beam. However, because the ceramic chamber is basically nonconductive, the rapidly changing magnetic environment will not induce eddy-currents therein.
In the article "A Low Coupling Impedance Double Helix Structure for Use in a Ferrite Kicker Magnet," written by Salvatore Giordano that appeared in IEEE Transactions on Nuclear Science, Vol. NS-30, No. 4, August 1983, pp. 3496-3498, a double helix wound wire structure was proposed to overcome beam coupling impedance inside an ejection kicker magnet between the beam and the material of the magnet. The double helix wound wire, however, still allowed the external magnetic fields of the kicker magnet to penetrate itself. However, a double helix wound wire structure cannot be used with higher intensity beams, i.e., above 1 .mu.A for beam current, because the transverse impedance of the structure is not low enough. A radio-frequency shield for use in the fast cycling magnets of the SNS synchrotron, as described in the Bulletin of the Rutherford Appleton Laboratory (in Oxon, England) No. 12, Aug. 23, 1982, consisted of a cage framework of wires running parallel to the direction of beam travel with non-conducting frames for maintaining wire separation at regular intervals along the beam travel axis. While this framework provided good longitudinal conducting pathways for carrying image currents, it did not have any transverse conductivity, and was complex and expensive to produce.
Overall a need still existed for a vacuum chamber which combined the characteristics of a sufficiently low rf impedance to allow the carrying of high-frequency image currents to provide beam stabilization and yet at the same time a high enough low-frequency impedance to minimize eddy-current losses and minimize distortion of the applied magnetic field. If the vacuum chamber for guiding intense particle beams does not meet both of these requirements, beam instabilities result. These beam instabilities would eventually cause the beam to be lost. Furthermore, it is necessary that the inside of an insulating vacuum chamber have some electrical conductivity to prevent the build-up of static charge.